This invention relates to a process for producing low-molecular weight olefins, e.g., ethylene from heavy hydrocarbons, comprising a hydrogenating pretreatment and a subsequent thermal cracking of at least part of the hydrogenation product.
Light starting materials, that is, hydrocarbons with a boiling point below 200.degree. C., such as naphtha, are particularly well suited for cracking hydrocarbons in order to produce olefins. They result in high cracking yields and produce few undesirable byproducts.
The great demand for such suitable starting materials for cracking will probably cause a scarcity and price increases for these substances. For some time, therefore, an attempt has been made to develop methods which permit the favorable utilization of a starting materials having higher boiling points.
In principle, the utilization of starting materials with higher boiling points results in lower yields of valuable cracking products; moreover, hydrocarbon fractions having boiling points over 200.degree. C., and which can be utilized only with difficulty are being produced in increasing amounts. Still further difficulties arise because starting materials with higher boiling points result in increased coke and tar formation in the cracking plant. These products, which are deposited on the walls of the various conduit elements, such as pipelines and heat exchangers, necessarily impair heat transfer and can also cause flow constrictions. When such materials are used, it is therefore necessary to remove such deposits more frequently than when light hydrocarbons are used.
A method intended to solve this problem is known from U.S. Pat. No. 3,781,195 (DE-OS No. 21 64 951), in which heavy hydrocarbons are catalytically hydrogenated prior to the thermal cracking. As a result, the proportion of aromatic, and especially polycyclic aromatic, compounds in the starting material, which are the primary cause of the undesirable products of cracking, is reduced. A desulfuration of the starting material takes place as well. Other prior art includes modifications of hydrotreating pretreatment process, for example, assignee's U.S. Pat. Nos. 4,297,204, 4,256,871, 4,260,474, 4,324,935 and 4,310,409. (The terms "hydrogenation" and "hydrotreating" are used interchangeably.)